The sensor is taking a reading every minute and stores them within itself. It can only store 8 hours worth so the scanner or mobile phone has to be used at least once every eight hours to download those readings, just by bringing the scanner or mobile phone within a few centimeters of the sensor. It uses Near Field Communication to pass the data and there has been a rumour for some time that they will start to use Bluetooth to communicate between sensor and scanner/mobile. If you forget which I have done, especially at night, it just overwrites the oldest entry so you always get the last eight hours worth of one minute readings.